The reason why it's there is to stop you from spamming requests to the server.
BTW, it doesn't have to be duck. The only reason why that's the "famous" command is because that was what I used to troll @HackersDontWin with on stream once

I think this is an anti-advertisement system than an easter egg.
There are hacked clients that send messages to people in the hub. Memeplex has it and I got muted there because I was "infected" by it.
The way it works is simply that it puts in letters and presses tab so it searches for players. Just like @hgbf said, it doesn't have to be "duck". It can be "banana", "orangejuice" or just "sdmqlkfjqmskjfq".